In some DC to AC power conversion applications, one or more parameters of the DC input voltage to the converter is a required input variable to the converter's AC output power and power factor control process that can be implemented by hardware, software or a combination thereof. The DC input to the converter may be direct from the output of a DC power source or a DC link voltage. Consequently closed loop power control, and power factor control, is dependent upon the DC input voltage and inherent input voltage ripple. For example in a fuel cell power system, a fuel cell's DC output voltage that is inputted to a high frequency DC to AC converter can be used as the input to a proportional-integral-derivative controller that controls the converter's AC power quality output based on dynamic variations (ripple) in the fuel cell's DC output voltage as disclosed in the prior art (e.g., Yu Jin Song and Prasad N. Enjeti. “A High Frequency Link Direct DC-AC Converter for Residential Fuel Cell Power Systems,” in Proc. IEEE-PESC, 2004, pp. 4755-4761).
Furthermore known pulse width modulated (PWM) power factor controllers do not limit instantaneous power in a linear manner. If the switched current exceeds an over current threshold, the PWM controller truncates the gate drive pulse width to instantly limit current and protect the power circuit. This discontinuous current limit boundary distorts the desired AC current, which increases the total harmonic distortion (THD) of the desired AC output current during such current limiting conditions.
Known DC to AC power converter controllers may use an average current mode control, or peak current mode control, of the power converter. These traditional methods require voltage feed-forward information from the DC input voltage as a control input to the power control circuit. Therefore, ripple voltage and other dynamic transients in the input voltage affect the quality of the current delivered to the load connected to the AC output of the converter. Consequently the frequency compensated error amplifier used in the controller is typically designed to prevent input voltage ripple from modulating the power control circuit, which would increase harmonic distortion.
It is one object of the present invention to provide closed loop power control and power factor control for a single phase DC to AC power converter that does not require feed-forward information from the DC input voltage to the converter.
It is another object of the present invention to provide controlled power conversion with up to exact unity power factor into AC line loads from DC sources and DC voltage links that may dynamically vary in voltage performance characteristics including magnitude.
It is another object of the present invention to provide a power factor control method that bounds both AC output current and AC output power as required by the state of the output AC line load in order to produce a robust system, with current and power defined, linearly controlled and linearly bounded even during AC swells (surges) or sags (dips) that can lead to excessive transient stresses on the power converter switching components.
It is another object of the present invention to provide a power factor control method that results in low total harmonic distortion during current limit mode operation by linearly bounding both AC output power and current, thereby preserving power quality even when the control circuit is in current limit mode.